The present invention relates to the art of rheological measurements and more particularly relates to an improved rheometer of the extrusion plastometer type and to an improved rheological measuring method.
Various types of rheometers are known. Such devices are utilized in the testing of flowable materials to determine their rheological characteristics, such as viscosity. Representative of such devices are those disclosed in U.S. Pat. Nos. 1,506,617; 1,664,839; 1,919,921; 2,028,187; 2,834,200; 3,036,214; 3,209,581; 3,283,565; 3,559,464; 3,595,305; 3,681,980; 3,766,773; 3,805,598; 3,908,442; 3,930,403; 3,933,032; 3,935,729. Another such representative device has been described by D. E. Elliott and P. J. Chiesa in an article entitled "A New Foam Rheometer for Studying Fire Fighting Foams," Fire Technology, Vol. 12, No. 1, pp. 66-69 (1976).
One important form of rheometer which has gained wide acceptance, especially in the plastics industry, is the so-called "extrusion plastometer". The extrusion plastometer incorporates a chamber having a discharge orifice and means for controllably forcing material from the chamber through the discharge orifice. Thus, a charge of the material to be tested is loaded into the chamber and then forced out through the orifice. While the material is being forced out, the various paremeters of the system, such as the amount of pressure applied to the material and the amount of material discharged per unit time are controlled or monitored. From these parameters, and from the known dimensions of the discharge orifice, the rheological properties of the material may be determined. Various forms of extrusion plastometer have been described in ASTM test method D1238 (American Society for Testing and Materials, Philadelphia, Pa.) and in the following U.S. Pat. Nos.: 3,203,225; 3,242,720; and 3,360,986.
However, prior to the present invention, no extrusion plastometer which is truly satisfactory for measuring the rheological properties of a multi-component mixture has been available. Such mixtures are frequently encountered in industrial practice. For example, one commercially important mixture includes a polyolefin polymer, a nucleating agent and a foaming agent such as a monomeric halogenated hydrocarbon. Because the properties of such a mixture change with time from the moment that the foaming agent is mixed with the other components, it is desirable to measure the rheological properties of the mixture immediately after mixing. However, with the extrusion plastometers the prior art, it has generally been necessary to first mix the various components outside of the chamber and then load the mixture into the chamber. As will be readily appreciated, such a procedure may entail some delay between mixing and measurement of rheological properties, which may cause the properties to change and may result in an inaccurate determination.